Copy number variation and elevated genetic diversity at immune trait loci in Atlantic and Pacific herring

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Abstract Background Genome-wide comparisons of populations are widely used to explore the patterns of nucleotide diversity and sequence divergence to provide knowledge on how natural selection and genetic drift affect the genome. In this study we have compared whole-genome sequencing data from Atlantic and Pacific herring, two sister species that diverged about 2 million years ago, to explore the pattern of genetic differentiation between the two species. Results The genome comparison of the two species revealed high genome-wide differentiation but with islands of remarkably low genetic differentiation, as measured by an F ST analysis. However, the low F ST observed in these islands is not caused by low interspecies sequence divergence ( d xy ) but rather by exceptionally high estimated intraspecies nucleotide diversity ( π ). These regions of low differentiation and elevated nucleotide diversity, termed high-diversity regions in this study, are not enriched for repeats but are highly enriched for immune-related genes. This enrichment includes genes from both the adaptive immune system, such as immunoglobulin, T-cell receptor and major histocompatibility complex genes, as well as a substantial number of genes with a role in the innate immune system, e.g. novel immune-type receptor, tripartite motif and tumor necrosis factor receptor genes. Analysis of long-read based assemblies from two Atlantic herring individuals revealed extensive copy number variation in these genomic regions, indicating that the elevated intraspecies nucleotide diversities were partially due to the cross-mapping of short reads. Conclusions This study demonstrates that copy number variation is a characteristic feature of immune trait loci in herring. Another important implication is that these loci are blind spots in classical genome-wide screens for genetic differentiation using short-read data, not only in herring, likely also in other species harboring qualitatively similar variation at immune trait loci. These loci stood out in this study because of the relatively high genome-wide baseline for F ST values between Atlantic and Pacific herring.

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